This invention relates generally to the medical imaging field, and more specifically to a method and system using ultrasound for imaging of blood flow, or other dynamic systems (tissue motion) through the human skull bone, or through other tissues (muscle, fat, etc.) in the medical imaging field.
All existing ultrasonic blood flow imaging algorithms exploit the phenomenon of ultrasonic reflection from the moving speckles in the blood (e.g. erythrocytes, air bubbles etc). Doppler methods are most effective when ultrasonic waves propagate parallel to the blood flow. Speckle tracking, speckle interferometry, and B-Flow methods are direction-insensitive. All listed methods are capable of overcoming the most common imaging obstacles, such as relatively weak useful echoes compared to the blood vessel wall reflections, and attenuation of acoustic waves by soft tissues. Imaging through the skull, however, is a more serious problem, and here all listed methods have their respective shortcomings.
Transcranial ultrasonic imaging of the blood flow is hampered by scattering, attenuation, and multiple reflections of acoustic waves at the surface and inside the skull. These effects tend to attenuate and distort the ultrasonic field transmitted through the skull, causing image quality degradation and resolution loss. Human adult skull bone is inhomogeneous, comprising three principal layers. The outer and inner layers, which are present not only in adults but also in children and animals, are composed of compact bone. A middle porous layer that is only present in adult humans turns out to be the main contributing factor to distortion of transcranial ultrasonic waves. Existing attempts to grapple with these problems are either invasive or require high-power ultrasound, which could harm the brain tissue. At clinically acceptable power levels and at relatively low ultrasonic frequencies allowing skull penetration the speckle reflections and even blood vessel wall echoes are typically buried in the noise, so that the amplitude-based algorithms fail. One existing algorithm that could potentially handle this issue is depth-insensitive and would produce a 2D projection of all blood vessels on the imaging (array) plane.
Thus, there is a need in the medical diagnostics field to create an improved method and a practical system using ultrasound for full 3D imaging of blood flow, or other dynamic systems (tissue motion) through the human skull bone, or through other tissues (muscle, fat, etc.) in the medical imaging field. This invention provides such an improved method and system.